With some physiological states, such as septic shock or post-coronary artery bypass surgery recovery, fluid therapy can be used to improve or preserve cardiac performance, and even mitigate imminent death. Where fluid therapy is utilized, the clinician can manage patient fluid intake based on physiological parameters such as arterial pressure pulse variation (PPV) and stroke volume variation (SVV). These parameters have allowed the clinician to anticipate the response of the patient to the fluid therapy.
Traditionally, both PPV and SVV have been calculated from invasively obtained measurements of stroke volume (SV), blood pressure (BP), and/or respiration. For example, PPV has been calculated based on the difference between measured pulse pressure maximum (PPmax) and measured pulse pressure minimum (PPmin) over the respiratory cycle, where pulse pressure has been manually determined for example, based on the difference between invasively measured systolic and diastolic pressures from heart beat to heart beat. SVV has been calculated based on systolic pressure variation (SPV), which has been calculated based on the difference between an invasively measured systolic blood pressure maximum (SBPmax) and an invasively measured systolic blood pressure minimum (SBPmin), over the respiratory cycle.
Unfortunately, as noted above, both the SV and BP values used to calculate PPV and SVV have been measured invasively. In addition, fluctuations in SV, for example, due to an irregular sinus rhythm or otherwise, may introduce error into the calculations. Furthermore, the respiration data taken during PPV and SVV has been measured under controlled mechanical ventilation, or breathing controlled via a ventilator, and spontaneous breathing may also introduce error into the PPV and SVV results. Furthermore, those calculations have been performed manually by a human (hand calculated), and are not easily plotted, averaged, or trended. Moreover, the hand-calculated results are not real-time values calculated with real-time measurements. As a result, the calculated results may not represent the current state of the subject.